There are six strings on the guitar. The thinnest one, farthest away from you, is referred to as the first string. This is the high E string. The next thinnest string is the B string. The next one closest to you is the G string. Then come the D string, the A string, and finally the fattest string on the guitar, the low E string.
The fretboard is the series of divided squares all along the neck of the guitar. The first fret is the one closest to the head of your guitar. As the numbers go higher, the frets get farther from the neck. For example, the first fret with a dot on it is the third fret, while the fret way up the neck of the guitar, with two dots on it, is the 12th fret.
Guitars that are built for left-handed playing are convenient, but if you have a standard guitar already, a luthier (a stringed-instrument craftsman) can convert it for you. Depending on your instrument, the luthier may need to replace your guitar's nut and bridge before he can re-string your guitar so that your first (high E) string is on the bottom when you hold it left-handed. It's not a difficult process, and many lefty players, including Jimi Hendrix, have played re-strung right-handed guitars. Now you're ready to play.
This is usually the first chord taught when learning guitar. Leave the low E string open. Put your middle finger on the second fret of the A string. Now put your ring finger on the second fret of the D string. Finally, put your index finger on the first fret of the G string. Leave the rest of the strings open. That's an E major chord.
Put your index finger on the second fret of the G string. Then put your ring finger on the third fret of the B string. Finally, put your middle finger on the second fret of the high E string. Voila---you're playing a D major chord.
Put your ring finger on the third fret of the A string. Put your middle finger on the second fret of the D string. Leave the G string open, and put your index finger on the first fret of the B string. When strumming this chord, do not touch the low E string. Play only the first five strings.
Place your middle finger on the third fret of the low E string. Place your index finger on the second fret of the A string. Leave the G string and the B string open. Finally, place your pinkie on the third fret of the high E string, and you have a G major chord.
The A minor chord looks exactly like the E major chord, but you move all your fingers up one string---one string, not one fret. Put your middle finger on the second fret of the D string. Place your ring finger on the second fret of the G string, and place your index finger on the first fret of the B string. Wow with this chord, as with the C major chord, you only want to play the first five strings. Do not hit the low E string.